For many years, the PET bottles usually found on the market have been manufactured by the blow moulding or stretch-blow moulding of PET preforms using compressed air.
A preform usually takes the form of a cylindrical tube closed at one of its ends and open at its opposite end. The open head of the preform corresponds to the neck of the container. During the conventional process for manufacturing containers from preforms, the preforms are slipped onto the cylindrical mounts of a continuous conveyor chain, which thus transports the preforms through an oven, essentially formed by a straight section bordered on each side by radiative heating means, so as to temperature-condition the plastic for the subsequent stretch-blow moulding step.
The hot preform is then taken off and transported into a mould of a blow moulding machine. The transport movement, performed for example by a transfer arm, is coordinated with that of the blow moulding machine, which is generally produced in the form of a rotary carousel that rotates continuously about its vertical axis and carries, on its periphery, a series of identical moulds. Thus, the preform is placed in the mould immediately after it has been opened and the previously formed container has been removed.
The preform is heated beforehand so as to be in the mould at a temperature above the glass transition temperature (about 100° C.) so as to enable it to be formed by stretch-blow moulding. The temperature of the preform at the end of the heating step is slightly above that required inside the mould of the blow moulding machine, so as to take into account the cooling that takes place over the distance that exists between the heating site and the blow-moulding site. Thanks to the simultaneous presence of several moulds, such a blow moulding machine can produce containers at very high rates, of around several tens of thousands of units per hour, i.e. around 1000 to 2000 bottles per hour per mould.
The stretch-blow moulding takes place by stretching using a metal rod and by injecting air at pressures ranging from 3 to 40 bar (3×105 Pa to 4×106 Pa). The air is injected through a nozzle, the end of which is introduced through the opening in the head of the preform.
Bottles manufactured by injecting pressurized air have a relatively satisfactory lifetime. However, the intrinsic properties and characteristics of PET, in particular its specificity which is to crystallize on being stretched, mean that even better results may be envisaged by modifying the container manufacturing process. In particular, it is known that high crystallinity means better mechanical strength.
The present invention makes it possible to extend the average lifetime of the containers within the context of normal transport, distribution and consumption. It also makes it possible to speed up the industrial production process, by optimizing the production steps and minimizing the in-line losses.
The invention applies preferentially to the class of polyesters in general, and to PET in particular.